1. Field of the Invention
This invention relates to a four-wheel steering (4WS) system for a vehicle. More particularly, the present invention relates to improvements for a rear wheel steering mechanism incorporated in a 4WS system.
2. Description of the Prior Art
As is well known, various attempts have long been made to improve controllability of automobiles. The 4WS system is one example which has resulted from such attempts.
In designing the 4WS system, the following points must be taken into consideration.
(1) High speed driving rarely requires large angle rotation of the steering wheel because such driving is used principally for driving on straight roads or in gentle curves.
(2) Low speed driving, on the other hand, often requires large angle rotation of the steering wheel to make steep curves or U-turns with a small turning radius.
(3) When driving in a corner or curve particularly at high speeds, the car is subjected to a large centrifugal force which may cause lateral slippage.
(4) Lateral slippage of the car can be prevented or reduced by orienting or pivoting the rear wheels in the same direction as the front wheels. Such a pivotal direction for the rear wheels will be hereafter called "same phase direction".
(5) The turning radius of the car can be reduced by orienting or pivoting the rear wheels in the opposite direction relative to the front wheels. Such a pivotal direction for the rear wheels will be hereafter called "opposite phase direction".
In view of these points, Japanese Patent Application Laid-open No. 61-9374 discloses a vehicle 4WS system incorporating a rear wheel steering mechanism which is capable of pivoting the rear wheels first in the same phase direction as the steering wheel is rotated from a neutral steering position, and then in the opposite phase direction when the steering wheel is rotated past a predetermined steering angle. Specifically, the rear wheel steering mechanism comprises an input shaft rotated in response to rotation of the steering wheel, and an eccentric shaft carried by the input shaft for rotation therewith. The steering mechanism further includes a stroke rod movable widthwise of the vehicle for pivoting the rear wheels, and a means for converting the eccentric or cranking rotation of the eccentric shaft into widthwise movement of the stroke rod.
For clearer understanding of the above-described rear wheel steering mechanism, reference is now made to FIG. 7 of the accompanying drawings which shows the pivotal angle curve for the rear wheels in relation to the rotational (steering) angle of the steering wheel.
As shown in FIG. 7, with the above-described rear wheel steering mechanism, the rear wheels are pivoted in the same phase direction when the steering wheel are rotated within a relatively small angular range AR from the neutral steering position, whereas the rear wheels are pivoted in the opposite phase direction when the steering wheel is rotated past that angular range AR. The same phase pivotal movement of the rear wheels within the small angular range AR of the steering wheel is utilized to prevent the vehicle from laterally slipping or excessively yawing (rotation about a vertical axis) in cornering or lane-shifting during medium- or high-speed driving. On the other hand, the opposite phase pivotal movement of the rear wheels at a larger rotational angle of the steering wheel is utilized to reduce the turning radius of the vehicle during low-speed driving, thereby facilitating making a U-turn or guiding the vehicle into a garage.
The prior art rear wheel steering mechanism described above is disadvantageous in the following three respects.
First, since the prior rear wheel steering mechanism converts the cranking rotation of the eccentric shaft into widthwise movement of the stroke rod, the pivotal angle of the rear wheels varies substantially according to a sine curve, as shown in FIG. 7. Therefore, in any rotational (steering) position of the steering wheel within the relatively small angular range AR, the rear wheels are always held pivoted in the same phase direction.
The same phase pivotal movement of the rear wheels, through effective for preventing lateral slippage of the vehicle in cornering at a high speed, has been found to provide a tendency of under-steering. Thus, with the prior art rear wheel steering mechanism, the driver must continue cornering with the tendency of under-steering as long as the steering wheel is kept in the angular range AR to hold the rear wheels pivoted in the same phase direction.
Indeed, the rear wheels should be pivoted in the same phase direction for entering a corner or curve at a high speed to prevent the vehicle from lateral slippage. However, one the vehicle has entered in the corner or curve, the rear wheels should be pivotally returned to their neutral position (or nearly neutral position) for continuation of cornering in a steady state, thereby avoiding under-steering.
Second, with the prior art rear wheel steering mechanism, the rear wheels are pivoted immediately in the opposite phase direction when the steering wheel is rotated beyond the relatively small angular range AR. In running at a medium speed, if often occurs that the steering wheel is rotated past the relatively small angular range AR for cornering along a relatively steep curve (hairpin curve) or an S-shaped curve particularly when driving up or down mountain roads. The opposite phase pivotal movement of the rear wheels during such medium-speed driving provides a tendency of over-steering, which also deteriorates the controllability of the vehicle.
Third, the same phase pivotal movement of the rear wheels prevents not only lateral slippage of the vehicle but also yawing of the vehicle. Although excessive yawing must be avoided to prevent the vehicle from spinning, a certain degree of yawing is necessary to suitably orient the vehicle for cornering or lane-shifting. Otherwise, the driver finds it strange that the orientation of the vehicle does not change even if he (or she) rotates the steering wheel properly. The driven then tries to further rotate the steering wheel in an attempt to correct the vehicle orientation. As a result, the front wheels are excessively pivoted.
With the prior art rear wheel steering mechanism, the rear wheels are immediately pivoted in the same phase direction when the steering wheel is rotated from the neutral steering position (zero steering angle), as shown in FIG. 7. Thus, yawing of the vehicle is restrained from the very beginning of cornering or lane-shifting, consequently failing to suitably orienting the vehicle.